1. Technical Field
The present invention relates to a microwave generator and an apparatus using the same.
2. Related Art
The Industry Science Medical (ISM) band that takes advantage of microwave is utilized in a variety of apparatuses including heating units, plasma generating devices, communications equipments and radar systems. In order to radiate such microwave, some microwave generators use magnetron for the oscillating source.
JP-A-9-265914 is a first example of related art disclosing a magnetron apparatus that includes a high-voltage noise filter. According to the example, a small and low-cost high-voltage noise filter can be obtained by: providing an insulating layer and a conductive layer on the surface of a coiled conductors providing another layer having a high withstand voltage between the conductive layer and the outer peripheral surface of the insulating layer near the end of the conductive layer, thus alleviating the electric field concentration and improving the withstand voltage characteristic of the insulating layer; and, in addition, reducing the thickness of the insulating layer,
JP-A-2004-265611 is a second example of related art disclosing a plasma processing device. According to the example, the plasma processing device uses a high-frequency source that is composed of magnetron, and the like.
FIG. 8 is a diagram showing the relation between the frequency and the strength in signals outputted from the magnetron. Here, the axis of abscissas represents the frequency and the axis of ordinate represents the strength. Besides outputting a specific frequency f2 that is needed for generating microwave, the magnetron used for generating microwave also outputs some other frequencies existing around the specific frequency f2. In other words, frequency signals outputted from the magnetron has a bandwidth. Therefore, for example, when a specific frequency f2 of 2.45 GHz is required, the magnetron also outputs some other frequencies that exist around 2.45 GHz. Consequently, the magnetron generates unwanted radiation, thereby giving rise to a problem that other apparatuses are adversely affected in such a way that they become incapable of communicating wirelessly in using the ISM band.
Also, since magnetron is large in size, it has so far hindered reduction in the size and weight of microwave generators using magnetron. In recent years, on the contrary, reduction of size and weight is increasingly required for some apparatuses using a microwave generator, and this, in turn, also requires microwave generators to become reduced in size and weight. However, microwave generators using magnetron for the oscillating source have been incapable of meeting this requirement. In addition, magnetron has also had some other problems including a significant amount of power consumed, poor frequency temperature behavior, and instability in outputted frequency, and so on.
Apart from the use of magnetron, an LC oscillator or a dielectric oscillator may also be used for the oscillating source in a microwave generator. Or, the frequency signals outputted from the oscillating source may be converted into high frequency signals through a phase-locked loop (PLL) circuit or a frequency multiplier circuit, to be used in a microwave generator. However, LC oscillators and dielectric oscillators have had such problems as poor frequency temperature behavior, instability of outputted frequency, a significant amount of jitters and variability in the frequency between each oscillator. On the other hand, PLL circuits and multiplier circuits have had problems including their large size that hinders size-reduction, the significant amount of power they consume and the significant amount of time they take in order to output the required frequency. Furthermore, PLL circuits have an additional problem that they are incapable of outputting a required frequency if any unlocking occurs.